Saccades in Huntington's disease: slowing and dysmetria.

Eye movements were recorded from 20 mildly affected patients with Huntington's disease (HD) who were divided into two groups, 10 patients with onset of symptoms before age 30 and 10 with onset of symptoms after age 30. In the younger onset group (HD less than 30), peak saccade velocities were low (less than 255 deg/sec for 20-deg saccades) in six of the 10 patients, whereas none of the 10 patients in the older onset group (HD greater than 30) had peak saccade velocities lower than 300 deg/sec. Latencies for volitional saccades were greater than normal in the HD greater than 30 group, but were normal for the HD less than 30 group. The ability to maintain steady fixation in the face of a distracting visual stimulus was decreased, to the same degree, in both groups of HD patients. In addition, 70% of the HD less than 30 group had an affected father, while 70% of the HD greater than 30 group had an affected mother. These findings suggest that the pathophysiology of the slow saccades, initiation deficit, and excessive distractibility in HD are different.